Dehydrogenation of isopentane to isoprene



May 2, 1961 J. R. OWEN DEHYDROGENATION OF ISOPENTANE TO ISOPRENE May 2, 1961 J. R. OWEN DEHYDROGENATION OFIISOPENTANE TO ISOPRENE Filed Nov. 21, 1957 2 Sheets-Sheet 2 .4T TORNEVS.

United States PatentfO .'DEHYDROGENATION OF ISOPENTNE TO ISOPRENE James R. Owen, Bartlesville, '0kla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed Nov. 21, 1957, Ser. No. 697,996

2 Claims. (Cl. 260`680) This invention relates to the dehydrogenation of isopentane to isoprene. Heretofore, it has been proposed to form isoprene directly from isopentane by` a single step dehydrogenation process. Considerable difficulty is encountered in obtaining adequate yields, and in making proper separations of the diverse products resulting from the single step dehydrogenation.

.I have discovered that isoprene can bemanufactured on a commercial scale ,by a'two-stage catalytic dehydro genation process. In the first step, isopentane is dehydrogenated in the presence of an alumina or magnesia catalyst promoted with an oxide of a metal of group IV, V, or VI of the periodic table. The resulting methyl butene is, in turn, dehydrogenated in the presence of a catalyst composed of a major amount of a potassium compound, a minor amount of iron oxide, anda small amount of chromium oxide. In this manner, isoprene is formed in suitable yields by a commercially feasible process.

A marked contribution to successful commercial operation is made by the particular separation steps utilized. More particularly, the eifluent from both dehydrogenation steps is combined, after removal of methane and light gases, and this combined product is fractionated'to provide an overhead product containing a preponderance of the isoprene present together with isopentane, 3methyl `ice `can be eiciently and economically carried out on a l commercial scale torproduce a valuable isoprene product.

Various other objects, advantages and features of theinvention will become apparent from the following detailed description taken in conjunction with the accompanying drawing in which -the figure is a Yow diagram of the dehydrogenation process .of the invention.

Referring now to the drawing in detail, the isopentane feedstock isfed through a line 10 to an isopentane dehydrogenation zone 11 Where it Viscontacted-with a catalyst under conditions such that .themajor reaction is the conemployed is magnesia, alumina, or a combination thereof promoted with up to 40 percentof an oxide of a metal ofgroup IV, V and VI of the periodic table. Specific examples ofsuch catalysts are alumina promoted with 40 ypercent 'chromium oxide, alumina promoted with 40` percent zirconiumoxide, alumina promoted with 40 percent titanium oxide, alumina promoted with 40 percent tin oxide, magnesia promoted withv percent molybdenum oxide, magnesia promoted with percent zirconium oxide, magnesia-alumina promoted with 20 percent vanadium oxide, and unsupported active chromium butene-l and 2-methyl-butene-1 together with a bottom product containing Z-methyI-butene-Z and the remainder of the isoprene. The overhead product is fed to a second fractionation zone where the S-methyl-butene-l is taken overhead, while the bottom product is passed to an ab-r sorber-stn'pper unit to separate isopentane, which is recycled, from a fraction consisting principally of isoprene and 2-methy1-butene-l. The latter fraction is combined with the isoprene, 2methyl-butene2 bottom product from the iirst fractionation zone and fed to an absorberstripper unit, from which an isoprene product together with any heavier materials present in the dehydrogenation eiuent is produced, the isoprene product being readily separated from the heavier oils in a subsequent fractionation step. An overhead fraction consisting principally of 2methylbutenel and 2methylbutene2 is recovered from the last-mentioned absorber, combined with the 3methylbutenel fraction from the second fractionation zone and fed to the second dehydrogenation step wherein these methylbutenes are dehydrogenated to stripper 21.

oxide. The temperatures and pressures are selected withina range suitable for the catalyst used.

For successful commercial operation, l utilize an alumina catalyst with 20 percent chromium oxide with the feed to the dehydrogenation step in dry conditions. Conversion temperatures range from -1000 to 1100 F. at a liquid hourly space velocity of 1 to 10. Pressure is not a critical variable, and the process can bey suitably operated at atmospheric pressure. However, -in both steps, use of sub-atmospheric pressure results rin increased isoprene yield.

The eluent from the dehydrogenationstep 11 is passed to a llight gas removal column 1-2 wherein methane, hydrogen, and other light -gases are taken overhead. The bottom product from the demethanizer column is fed to a fractionation zone 13. l

In the zone 13 a cut is made at isoprene, thus providing an overhead fraction of 3-methyl-butene-1, isopentane, 2-methy1-butene-l and most ofthe isoprene together with a bottom product of Z-methyl-butene-Z and the rest of the isoprene.

The overhead product from the zone 13 is fed to a fractionation `zone 14 whence B-methyl-butene-l overhead product is withdrawn by a line 15.

The bottom product from the zone 14 is fed to a unit consisting of an absorber 16 and a Istripper 17. In this unit, the bottom product from the fractionator 14 is contacted With a suitable non-volatile absorptive material, for example, furfural. Many solvents can be used in this step in a broader aspect ,of the invention, as for example, ethylene glycol, methyl carbitol, and methyl Cellosolve. An Voverhead isopentane product is obtained from the absorber 16 which is recycled through a line 18 to the feed conduit 10 of the isopentane dehydrogenation zone. Richsolvent from the absorber 16 is passed to the stripper 17 whence a fraction consisting principally of isoprene and 2-inethylbutenel is Withdrawn by a line 19. This fraction iscombined with the isoprene, Z-methyl-butene-'Z bottom product from the lfractionation zone 13 and fed to a unit consisting of an absorber Z0 and a In this unit, there is circulateda solvent vPatented May 2, 1961.

. oxide, 30 percent potassium carbonate and the balance of the type previously described in connection with the absorber-stripper unit 16, 17. An isoprene product containing any heavier materials which may be present is prene product is withdrawn overhead through a line 24 while the heavier oils are recovered as a bottom product.

byaline25. Y

The overhead product, of the absorber 20, which consists principally of 2methylbutene1 and Z-methyl-butene-2, is withdrawn through a line 26, combined with the B-methyl-butene-l overhead product from the fractionation zone 14 and fed to an olefin dehydrogenaton unit 27. In some cases, it is desirable to combine a portion of the bottom product from the fractionation zone 13 with the feed tothe dehydrogenaton unit 27.

In the unit 27, themethylbutenes are dehydrogenated to isoprene by contact with a catalyst under suitable conversion conditions'. As most catalysts of this type are steam active, steam is mixed with the feed by a line 28.

Inga broad aspectV of the invention, any suitable olefin dehydrogenaton catalyst can be employed inthe zone 27, such as a catalystcomposed of 3 percent chromium' iron oxide. n

However, an outstanding catalyst for securing commercially ,satisfactory yields of isoprene is ,a potassium base material composed of 51 to 59 .percent by weight potassium carbonate, 39 to 47 percent by weight iron oxide and 1 to 10 percent by weight chromium oxide. With this particular catalyst, satisfactory conversion of methylbutenes to isoprene is obtained at temperature of 1100 toV The effluent from the zone 27 is fed to a light gas reY moval zone 29 where methane, hydrogen and other light gases are taken overhead, the bottom product being fed through a line 30 to the fractionation zone 13 in combination with the vbottom product from the zone 12. If steam is fed to the reactor 27, a condenser and settler for water should be interposed between the reactor and the zone 29. Thus, it will be apparent that decided economies in separation are achieved by the process of the invention in that separate fractionation systems are not required for the two dehydrogenaton steps. Moreover; the separation process permits the recovery of substantially pure isoprene together with recovery vof isopentane and methylbutenes of .suitable purity for recycling to'the respective dehydrogenatonY zones 11` and .27. The key separation is in'the absorber column 20. At a tempera-` ture of 25 C., a water content of 16 percent by volume and a hydrocarbon content of 6.2 weight percent in the solvent phase, a relative volatility Vof 2methyl-butene-2 to isoprene of 1.82 can be attained with furfural solvent which contributes to the vrecovery of .the pure isoprene and separation of the methylbutenes in suflicient purity to recycle. Many pieces of apparatus are thuseliminated from the separation system without losing the function of such apparatus. Also, the two-stage dehydrogenaton process of is'opentane to isoprene utilizing a chromium oxide catalyst supported on alumina for the first-stage andthe potassium oxide base catalyst containing iron oxide and a small4 amount of chromium oxide in the second dehydrogena-V tion stage has been found to provide economically suitable results'and yields for commercial operation.

` The following is presented as a specific example of my process on the basis of converting 100 mols of isopentane. he compositionvof the materials atdiiferent portions of thcew Wares fellows f f. 4

Feed to The Feed to Isopen- Eiuent From Process (Line tane Dehydro- Isopentane De- 10) genation Zone hydrogenation 11 Plus Recycle Zone 11 Component Mol Mol Mol Mols Per- Mols Per- Mols Percent cent cent Hydrogen..- 72.0 19.3 Methane 4. 8 1. 3 Ethylene..V 1.1 0.3 Ethan@ 3. 0 0. 8 Propylene. 3. 0 O. 8 Propane... 1.5 0.4 Bntm-m 2. 2 0. 6 Bntane 0. 7 0.2 Isoprene 18. 3 4. 9 B-Methyl-buteno-l.- 12. 7 3. 4 2-Methyl-butene-1.. 19. 8 5. 3 2-Methyl-butene-2. p 35. 0 9. 4 Isopentane 100. 0 100. 0 297. 0 100. 0 197. 0 52. 9 Oils.(as CmHm)- p 1.5, D. 4

There is alsoV recovered fromV the dehydrogenaton zone 17.4 mols of coke, calculated as CHUM.

- Overhead Product Bottom Product of Zone 12 from Zone 12 Component Mols Mol Mols Mol Percent Percent Hydrogen 72.0 Methane 4.8 Ethylene 1.1 Ethane 3.0 Propylene 3. 0 Propane 1.5 Butene 2. 2 Butane. 0.7 Isoprene. 18. 3 6. 4 B-Methyl-butene-l l2. 7 4. 5 2MethylButene-1- 19. 8 6. 9 2Methyl-butene-2. 35. 0 12. 3 Isopentane 197.0 Y 69.4 Oils n 1. 0.6

Feed to Overhead Bottom v l 'Fractionatlon Product From Product From Component Zone 13 Fractionaton Fractiona-tion Zone 13 Zone 13 Mols Mols Mols Mols Mols Mols Percent Percent y Percent Isoprene..; 79. 6 34 7 B-Methyl-butene-l.. 20. 1 2Methyl-butene1 53. 6 Z-Methyl-buteno-Z.- 92. 5 64. 3 Isopentane 197. 0` 44. 5 197. 0 65. 7 Oils..` 1.5 0.3 1.5 1.0

Overhead Bottom Overhead Product From Product From Product From Fractionatlon Fractionation Absorber 16 Component Zone 14 Zone 14 l Mols Mols Mols Mols Mols 'Mols Percent Percent Percent Isoprene 3-Methyl-butene-1.. Z-Methyl-butene-l.. Isopentane werelas follows, furzurall being usedgas the! absorbent P Orar@ am' essere mmh T 110 [0111 e y Ogena 8 l1 e y Component Stripper 17 tion Zone 27 drogenation 2011627. .1 u l y Top Tem- Pressure, M015 Mois Mols' Mois' MoisY .M515 v separati Unit Peame'l P-Si'a' Percent Percent Percent Fraetionation'zonegls 130 130 84 ffn" sgg 3H' Freouonanon zone 14. 130 140 a9` Ethlener" d5* j0'2 lol Absorber 16-- 80l 80 il) o Y 6:6 2-6. Str1pper17-- 175v v320 00 Profp- 1pm 0-8 l 1o-3v Absorber 20 80 so (1) BWK., 3:9 1; 5f Stripper'm 175 820 a0 Bumdieno 0:8 l 0:3, Isoprene Column 23 v 135 201 30 Isoprene 29.6 35.6 61.3 e 24.0 eillyi'uienefi" 53 6 644 2(3)0 :im 32;'4 ig Liquld Phase' e yl1e11e 5. 2.2 .8 .2` a-Methyl-butene-z-- 92.5 55.7 57.5 y 22,5 Of c0urse,-depend1ng upon the particularsolvent used 88 2 100 0- 166 2 100 0 2557 m0 o and on the stream composition, the temperatures and f. pressures utilized in the separation may vary toproduce the desired overhead and bottom products previouslyA Ovierhead PZrod- Botltom Pod#` Feedbto degcribed 110 Tgfin one et rggi one Sor er o 2O W1th respect to the 1sopentane dehydrogenatiomno ap- Component preciable spoiling of the' catalyst is observedwithpen- M01 M01 I M01 tane feed stock overextended'periods of operation'. Dur- Mols eenrt Mols ggf; Mols ggf; ing the conversion.' a carbonaceous deposit accumulates on the catalyst and, when 2.0 to 2.5 weight percent of Hydrogen 80 3 840 25 coke is present, the catalystl should ber regenerated by Methane. 2.8 29 burning the. coke therefromwith combustion gaseslcon- Ethzylfnef g2. g-g taining 3 rto 5 percent oxygen.v One hour process and Propylene 0.8 0.8 one hour regeneration cycles are suitable for large scale gf- 1 production and, under the described conditions, average Isoprene conversions as high as 33 percent can be readily obtained. jgjg: With the described catalystoperating under the described 2-111eth'y1-bntene-2-- conditions, the reaction is quite selective, yielding hydro- 011s-- -7 gen and methylbutcnes plus isoprene with only minor 95.7 100.0 v amounts of C1 to C4 parans |and olens and only'traces Y of products having 6 or more carbon atoms. `Surpris- Overhead Overhead Bottom Overhead ingly, no straight chain methylbutenes are'formed which Productor' Product of 'Productor Productor would yield the straight chain diolen piperylene rather' Absorbeo Stripper 21 -crffa clffl' than the desired product. isoprene.r Infrared datav indi- Component 40g cate that no straight chain amylenes are formed, with the M01 M01 M01 M01 sensitivity of the instrumentI being 0.25 percent'. Mols VPer Mols vPeuri Mois Per Mo1s Pert-I The following example'indicatesthe products obtained' een ce mn een and conversions at various reaction conditionsl in zone l1-` gsgreie.. 56.55; 79.0 98.2 79.01000 TABLE II et .yu e f f 2Methy1butene 02.5' 63. 5v Products and conversions at varzous reactzon condztzons ons- 1.5 1.8. 1.5 100.0

146.1 100.0 81.1 100.0 1.5 100.0 79.6 100.0 Run A B c f `1,000 1,025 1,050 A catalyst conslstmg of percent by weight chromium' 4. 9 4; 9 4. 9' oxide supported on alumina was utilized in the isopeng'lxjels $101335- 2761) 316g 3362 tane dehydrogenation zone 11, anda catalyst consisting Eiciemy('rsamy1`1|'1s 04.7 8725 8317 of 52.2 weight percent potassium carbonate, 44.6 welght Composition of Products Weight pep percent iron oxide and 3.2 weight percent chromium oxcent; v ide was utilized inthe olefin dehydrogenastion zone 27. g-g ggg ggg Conditions in the dehydrogenation zones were -as fol- 0. (114 0. '0.144'y 0. 1 0. 0. 0 lows' 0.14 0. a1 0. 61 0. 09 0.21 0. 32 0.01 0.02 Isopentane Olen De' 0. 09 '0. 49 0. 67

Der-1m- 15 5 15 gena 1011 1011 'i 60 3-Me1hy1-bu0ene- 3. 41 3.88 4.07 2Metl1yl-butene-l.. 7.55 7.62 6.56 Temperature, F 1. 050 1, 175 2Met1iy1butene2 11.85 12. 50 11. 52 Liquid hourly space velocity.- 5 2 Isopentane 72.90 68. 48 66.66 Mols steam/mol hydrocarbon- 0 10 Heaviers 0. 30 1. 00 on stream time (1) 2) coke 0. 43 0. 70 1. 01 Carbon balance conversiony1eld data Conversion, mol percent 33.65 40. 57 65 i 100.00 100. 00 100.00 Yield, mol percent 3 28. 76 36. 83 f selectivity, m01 prCent. o 85.47 90. 78 Conpittinltg lentelrms;` 15 16,0 18,2

e, y-uene- L 2-Methy -butene-1'. 33.1y 31.8 29.8 honr out; of each 2.1 2-Methy1-butene-2 01.9 52.2 52.0 on inuous. o1enns+d1o1ef1n. 70 100.0 100.0r 100.0 Dioleflrn y Yield of lroducts,lb./100lb.Isopentane l It will be noted that utilizing the vdescribed catalysts Rlfgen ughm Y3 6 9 1 m2 and conditions a yield of 79.6 mols of isopre'ne was ob Egglrrenes ggg tained for each 100 mols of fresh isopentane charged. Coke 116 212 31p,

Operating conditions for the various separation zones tion.Y The conversions obtained are 7 to 1l' percentage` points higherthan the conversions obtained in converting buteneto butadiene with the same catalyst..

Following are data indicating the excellent-conversions, yields, and efciencies at various operating conditions:

- TABLE 'III'V Composition ofproducts and yields Conditions:

' Temperature .sr- 1,175 1,175 1,200 Liquid Hourly Space Velocity. 4 2 2 Steam to Hydrocarbon 9. 77 9. 79 11. 93

Conversion Data:

Conversion, Mol'percent 31. 00 40. 25 '47. 20 Yield, Mol percent 29. 10 37.00 4.2.10 Emclency, vMol percent 93.80 92. 89. 40

Products: 1 i

Hydrogen 25. 24 31. 40 34. 72 Methane---" .81 1.12 1.56 Ethylene 19 29 Carbon Dioxide. 1. 81 2. 56 4. 00 Propyleue 22 33 44 Iso and n-Butene Y .78 1.26 1.93 t-Butene-2 .08 10 .14 c-Butene-2 05 08 09 Butadiene-1,3.- 22 33 47 3Methylbutene1. 3. 73 2. 92 2. 40 2Methyl-butene-1- 16. 50 13. 20 10. 62 2Methyl-butene2 29. 35 22. 50 18. 35 Isoprene 20. 95 23. 95 "25. 00

l Composite analysis by mass speetrographic and gas chromatographic methods. u

To further illustrate the broad aspects of the invention relating to the two-step dehydrogenation of isopentane to 'form isoprene, I have shown in Figure 2 a system wherein the dehydrogenation products are separated by liquidliquid extraction, rather than by fractionation. InLthisv modification, there is an isopentane dehydrogenation zone 31 corresponding to the zone 11 of Figure l, an olelin dehydrogenation zone 32 corresponding to the zone 27 of Figure l, and la light'gas removal zone '33."

In the modiiication of Figure' 2, the eflluents from both dehydrogenation zones are' combined and separated in' a single zone, rather than'in'separate zones as in Figure l. Q

In one specific embodiment ofthe invention, 422.8 mois per hour of effluent from the zone 33 is passed to a liquid-liquid extraction column 34 Where it -is-,con-Y tactcd with methyl carbitol as. a solvent.. Thesolvent enters the column through--a'line'SS -atv the rate'o 37,300 mols per hour, `and contains'lS. percent water..Y Rafinate is withdrawn from'the wtop of-the column 34 by line 36 and passed toa ash zone 37... 192.0V'rnols per hour of isopentane is withdrawn overhead from the flash zone 37 and passed through a" recycle linel Y318v to the dehydrogenation zone Y31.Y About l mol per hour of solvent passes from the ash zone 37 to the line 35 as recycle solvent. Y Y* Y u Y The extract from the column 34 is passed'. through a line 39 to a stripping zone 40, from which solvent is recycled to the column 34 by aline 41. The eiiluent from the column .is divided into `two portions,'4,710 mois per hour passing back to the column 34'through a line 42 asreflux, and 250.8 molsper `hourpassing through a line 43 to a second liquid-liquid extraction column 44.

` In the specific vembodiment described, the column 34.

has trays and isncperated at a temperature of 81 FQ The material balance for the column 34jis shown by the followingtable, the figures being expressed infmols per hour.

' 'In the second extractionY zone V44,-a further separationV is made to yield an isopene product and a methylburtenel fraction suitable` for 'feed to the dehydrogenation zone: 32.k To this end, 'the'ra-atefrornfthe'fcolumn 44 is passed throughla'linec'46 to a'flash zone-47: About i3. mols per'hourwof methyl carbitol solventV is withdrawn from zone. 47. by. a line 48 andhpassedgto the column 44" througha .line 492along` with8,400 mols per hour of recycle solvent containing 14.6 percent Water. An oleiinA fraction is withdrawn overhead from the ilash zone 47 at a rate of 162.6 mols. per hour and passed through a lline 50 to the olefin dehydrogenation zone 32.

The'extract from'the column 44 is passed through a line 51 to a stripping zone 52,7V from which recycle solvent is withdrawn and fed to the column 44 through the line 49,-as previously described. The overhead product fromthestripping zone 52 isdivided into'twoportions. 872

/mols per jhour'isY returned` as reflux to the-zone 44 by a line 53, and`79.2 mols per hour of isopr'e'ne isyielded as a product through a line 54. j r r j [n.the specific embodiment described, 'the Vcolumn 44 has 60 trays and operates atY a temperatureof 81 F.

The material balance for Vthe columnis shown by the following table:

Component Feed Rathnate Extract 3-Methyl-butene-1 19. 1 18. 9 0. 2 9. 0 9. 0 52. 5 52. 0 0. 5 79. 6 2. 0 77. 6 90. 6 89. 7 0. 9

A and light gases from the dehydrogenation eiliuent, fractionating the resulting material containing substantially all of the 5 carbon atom hydrocarbons contained in said dehydrogenation effluent tov separate a lirst fraction consisting essentially of 3-methyl-butene-1, isopentane, 2- methyl-butene-l land isoprene together with a second fraction consisting essentially of isoprene and 2methyl. butene-2, fractionatingsaid rst.,.fr ationnto separate 3- methyl-butene-l therefrom, contactingvthe remainder` of the material with an abso'rp'tonlmcdillmll a rst absorption zone, recycling i'sopentaney from said iirst. absorption zoneto the dehydrogenation step, stripping a fourth fraction consistingfessentially of isoprene and 2- methyl-butene-'lA from theY richY absorbent material, combining said second and fourth fractions and contacting them with an absorption medium in a secondl absorption zone, recoering a fifth fraction consisting essentially of 2- methyl-butene-l and 2-methyl-butene-2 fromsaidsecond absorption zone, stripping an isoprene fractionfrom the rich absorbent material in'said second rabsorptionzone, separating an isoprene product from heavy oils in said isoprene fraction, combining said iifth fraction with said 3-methyl-butene-1 fraction, contacting the last-mentioned fractions with a catalyst consisting essentially of 51 to 59 percent by weight potassium carbonate, 39 to 47 percent by weight iron oxide and 1 to 10 percent by Weight chromium oxide at a temperature of 1100 to 1250" F. together with steam, separating methane and light gases from the efiiuent of the last-mentioned dehydrogenation zone, and passing the remainder of said ef'liuent oontaining substantially all of the carbon atom hydrocarbons contained in the effluent of said last-mentioned dehydrogenation zone to the first fractionation zone along with the demethanized efiiuent from the first dehydrogenation zone. Y

2. The method of producing isoprene which comprises contacting isopentane with a chromia-alumina catalyst comprising about 20 percent chromium oxidel at a temperature of 1000 to l100 F., separating methane and light gases from the dehydrogenation eluent, fractionating the resulting material containing substantially all of the 5 carbon atom hydrocarbons` contained in said dehydrogenati'on effluent to separate a first fraction consisting essentially of 3-methyl-butene-1, isopentane, 2- methyl-butene-l and isoprene together with a second fraction consisting essentially of isoprene and 2methyl butene-2,l fractionating said rst fraction to separate 3- methyl-butene-ltherefrom, contactingvthe remainder of the material with an absorption medium in a first absorption zone, recycling isopentane from said first absorption zone to the dehydrogenation step, stripping a fourth fraction consisting essentially of isoprene and 2methyl butene-l from the rich absorbent material, combining said second and fourth fractions and contacting them with an absorption medium in a second absorption zone, recovering a fifth fraction consisting essentially of 2- methyl-butene-l and 2-methyl-butene-2 from said second absorption zone, stripping an isoprene fraction from the rich absorbent material in said second absorption zone, separating fan isoprene product from heavy oils in said isoprene fraction, combining said fifth fraction with said 3-methyl-butene-1 fraction contacting the last-mentioned fractions with a catalyst consisting of about 52 percent by Weight potassium carbo-nate, about percent by weight iron oxide kand about 3 percent by weight chro mium oxide at a temperature of 1100 to 1250 F. together with steam, separating methane and light gases from the eluent of the last-mentioned dehydrogenation zone, and passing the remainder of said eiuent containing substantially all of the S-carbon atom hydrocarbons contained in the effluent of said last-mentioned dehydrogenation zone to the first fractionation zone along with the demethanized eiuent from the first dehydrogenation zone.

References Cited in the fleof this patent UNITED STATES PATENTS 

1. THE METHOD OF PRODUCING ISOPRENE WHICH COMPRISES CONTACTING ISOPENTANE WITH A CHROMIA-ALUMINA CATALYST AT A TEMPERATURE OF 1000 TO 1100* F., SEPARATING METHANE AND LIGHT GASES FROM THE DEHYDROGENATION EFFLUENT, FRACTIONATING THE RESULTING MATERIAL CONTAINING SUBSTANTIALLY ALL OF THE 5 CARBON ATOM HYDROCARBONS CONTAINED IN SAID DEHYDROGENATION EFFLUENT TO SEPARATE A FIRST FRACTION CONSISTING ESSENTIALLY OF 3-METHYL-BUTENE-1, ISOPENTANE, 2METHYL-BUTENE-1 AND ISOPRENE TOGETHER WITH A SECOND FRACTION CONSISTING ESSENTIALLY OF ISOPRENE AND 2-METHYLBUTENE-2, FRACTIONATING SAID FIRST FRACTION TO SEPARATE 3METHYL-BUTENE-1 THEREFROM, CONTACTING THE REMAINDER OF THE MATERIAL WITH AN ABSORPTION MEDIUM IN A FIRST ABSORPTION ZONE, RECYCLING ISOPENTANE FROM SAID FIRST ABSORPTION ZONE TO THE DEHYDROGENATION STEP, STRIPPING A FOURTH FRACTION CONSISTING ESSENTIALLY OF ISOPRENE AND 2METHYL-BUTENE-1 FROM THE RICH ABSORBENT MATERIAL, COMBINING SAID SECOND AND FOURTH FRACTIONS AND CONTACTING THEM WITH AN ABSORPTION MEDIUM IN A SECOND ABSORPTION ZONE, RECOERING A FIFTH FRACTION CONSISTING ESSENTIALLY OF 2METHYL-BUTENE-1 AND 2-METHYL-BUTENE-2 FROM SAID SECOND ABSORPTION ZONE, STRIPPING AN ISOPRENE FRACTION FROM THE RICH ABSORBENT MATERIAL IN SAID SECOND ABSORPTION ZONE, SEPARATING AN ISOPRENE PRODUCT FROM HEAVY OILS IN SAID ISOPRENE FRACTION, COMBINING SAID FIFTH FRACTION WITH SAID 3-METHYL-BUTENE-1 FRACTION, CONTACTING THE LAST-MENTIONED FRACTIONS WITH A CATALYST CONSISTING ESSENTIALLY OF 51 TO 59 PERCENT BY WEIGHT POTASSIUM CARBONATE, 39 TO 47 PERCENT BY WEIGHT IRON OXIDE AND 1 TO 10 PERCENT BY WEIGHT CHROMIUM OXIDE AT A TEMPERATURE OF 1100 TO 1250* F. TOGETHER WITH STEAM, SEPARATING METHANE AND LIGHT GASES FROM THE EFFLUENT OF THE LAST-MENTIONED DEHYDROGENATION ZONE, AND PASSING THE REMAINDER OF SAID EFFLUENT CONTAINING SUBSTANTIALLY ALL OF THE 5 CARBON ATOM HYDROCARBONS CONTAINED IN THE EFFLUENT OF SAID LAST-MENTIONED DEHYDROGENATION ZONE TO THE FIRST FRACTIONATION ZONE ALONG WITH THE DEMETHANIZED EFFLUENT FROM THE FIRST DEHYDROGENATION ZONE. 